In the image forming apparatus such as a tandem type color copying machine utilizing an electrophotographic process, image forming sections, configured with a photoconductive substance drum, a charging device, an optical scanning device and a developing device, are provided for respective colors i.e. yellow (Y), magenta (M), cyan (C) and black (K), and are disposed along an endless intermediate transfer belt so that a color image, formed by overlapping the images of respective colors Y, M, C and K on the rotating intermediate transfer belt, is transferred onto a transfer sheet from the intermediate transfer belt.
As above, in case the color image is formed by overlapping the images of respective colors, a clear image cannot be obtained due to color shift unless forming positions of images correspond to each other correctly between colors. Therefore it has been carried out that a test image, so called a registration mark, for correcting color shift is formed on the intermediate transfer belt, and an optical sensor reads the mark to derive a necessary correction amount so as to correct the forming positions of images formed by image forming devices for respective colors where necessary (for example Patent Document 1: Unexamined Japanese Patent Application Publication No. H1-142679).
Also, correction control is carried out so that the intermediate transfer belt runs stably, and to prevent the intermediate transfer belt from meandering and a bias (for example Patent Document 2: Unexamined Japanese Patent Application Publication No. H9-48533).    Patent Document 1: Unexamined Japanese Patent Application Publication No. H1-142679    Patent Document 2: Unexamined Japanese Patent Application Publication No. H9-48533
Bias correction control of the intermediate transfer belt is usually carried out on a steady basis so as to avoid color shift due to meandering and the bias of the intermediate transfer belt. However, if bias correction of the intermediate transfer belt is carried out during color registration correction, an amount of color shift measured based on the registration mark becomes a sum of a color shift amount caused by shift of the forming position of the image formed by the image forming section and a color shift amount of the intermediate transfer belt caused by bias correction. Therefore, if the color registration correction is carried out based on the color shift amount measured, the correction amount can be incorrect.
On the other hand, if the bias correction control of the intermediate transfer belt is simply ceased during the color registration correction, the color shift amount caused by meandering or the bias of the intermediate transfer belt during the color registration correction is included in the color shift amount measured. Thus if the color registration correction is carried out based on the measurement value, the correction amount becomes also incorrect.